Use a depth buffer for the GL surface in FirefoxReality.

support/android/apk/servoview/src/main/java/org/mozilla/servoview/ServoSurface.java

@@ -136,7 +136,7 @@ public class ServoSurface {
                     EGL14.EGL_GREEN_SIZE, 8,
                     EGL14.EGL_BLUE_SIZE, 8,
                     EGL14.EGL_ALPHA_SIZE, 8,
-                    EGL14.EGL_DEPTH_SIZE, 0,
+                    EGL14.EGL_DEPTH_SIZE, 16,
                     EGL14.EGL_STENCIL_SIZE, 0,
                     EGL14.EGL_NONE
             };